To employ a computer in the analysis of a video signal, it may be necessary to first encode the video into a format that may be easily recognized and manipulated by the computer. This is accomplished via a video encoder. Therefore, the perceived quality of video that a set-top box outputs depends on the capabilities of the video encoder and the original video signal.
When the encoder digitizes a video program, the signal that the encoder generates may depend on the precise starting point, in the video program, at which the video server begins the encoding. This is particularly true for encoding schemes that employ temporal compression, such as MPEG2 and MPEG 4. Furthermore, Gaussian noise in the video stream will be different for different encoding sessions. Consequently, even from the same video program, it is unlikely that the video encoder will generate an identical video signal across two encoding sessions.
When set-top boxes of the same make and model receive an encoded video signal from the same video stream, the set-top boxes may generate different output video signals. There are many reasons for the differences. One reason may be that the set-top boxes use different decoding algorithms (e.g., different versions of a decoder). Another reason may be that the set-top boxes produce different amounts of noise in the output video signals. Yet another reason may be that, with respect to hue, color, and saturation, each set-top box may be calibrated differently from other set-top boxes. Yet another reason may be that each set-top box is designed to generate output video whose quality depends on a processing load of the set-top box, and the set-top boxes may be under different processing loads. For example, the set-top boxes may use a decoding algorithm in which rendering a particular frame depends on minute delays in processing.